Bicyclotetrasilazanes and the preparation thereof



United States Patent 3,518,290 BICYCLOTETRASILAZANES AND THE PREPARATIONTHEREOF Christopher A. Pearce, Cowbridge, Glamorgan, and Norman C.Lloyd, Radyr, Cardiff, Glamorgan, Wales, assignors to Midland SiliconesLimited, Reading, Berkshire, England No Drawing. Filed May 21, 1968,Ser. No. 730,940 Claims priority, application Great Britain, May 26,1967, 24,693/67 Int. Cl. C07f 7/02 US. Cl. 260448.2 11 Claims ABSTRACTOF THE DISCLOSURE Novel compositions of matter and processes for preparing bicycloand aminocyclotetrasilazanes. These compounds are usefulas starting materials for the preparation of silazane polymers, asintermediates in the preparation of silazane-siloxane copolymers and ascrosslinking agents for siloxane polymers.

This invention relates to novel organosilicon compounds wherein thereare present silicon atoms linked to nitrogen atoms and also relates to aprocess for the preparation of such compounds.

More particularly, this invention provides a novel com position ofmatter comprising a bicyclotetrasilazane of the general formula:

R'N NR NR SiN-S iR R R I IHR' wherein each R represents an alkyl radicalcontaining less than 6 carbon atoms, a monocyclic aryl radical, a vinylradical or an allyl radical and each R represents an alkyl radicalcontaining less than 6 carbon atoms or an allyl radical. Preferably eachR represents a methyl, ethyl or phenyl radical and each R represents amethyl or ethyl radical.

The novel bicyclotetrasilazanes of this invention can be prepared byheating, under substantially anhydrous conditions, a tris-(amino)silaneof the general formula RSi(NHR') wherein R and R have the significancenoted above. Methods of preparing the tris-(arnino)silanes are wellknown in the art. One convenient method comprises reacting anorganotrichlorosilane with the appropriate amine, for example, amethylamine or ethylamine.

In order to obtain the desired compound, the tris- (amino)silane shouldbe heated to a temperature above 210 C. and this invention includes sucha process. The actual temperature employed will depend mainly on thenature of the R groups and to some extent on the R groups. The largermolecular weight radicals in general require the use of highertemperatures. The reaction time required can be reduced by the use of acatalyst and the preferred catalysts being acidic materials such assulphuric acid, ammonium sulphate and alkylamrnoniu-m salts which arecapable of acting as a proton source at the reaction temperature. Ingeneral, the use of a catalyst in the reaction is preferred as thissignificiently reduces the reaction temperature in addition toshortening the reaction time.

Subject to volatility considerations, the formation of the desiredproduct can be followed by a gas chromatography technique thus enablingthe optimum reaction conditions to be obtained for the lower values ofR. As an 'ice indication of the temperatures at which thebicyclotetrasilazanes can be obtained we have found that when Rrepresents the propyl radical and R the methyl radical, thebicyclotetrasilazane is formed at about 330 C. when the appropriatetris-(an1ino)silane is heated in the presence of from about 0.005 to0.01 moles of catalyst per mole of silane. When R is the ethyl radicaland R is methyl, a temperature of about 300 to 320 C. is required toprovide the desired bicyclotetrasilazane in the presence of about 0.005to 0.010 mole of catalyst per mole of the tris-(amin0)silane. When R ismethyl, a correspondingly lower temperature can be used for the samecatalyst concentration.

In view of the susceptibility to hydrolysis of the siliconnitrogenlinkages in the tris-(amino)silanc, the reaction is performed undersubstantially anhydrous conditions, for example, under dry nitrogen orother inert gas.

The tris-(amino)silane can be dissolved in a solvent prior to theheating step. The presence of a solvent is not, however, essential andthe choice of a suitable organic solvent can be diflicult in view of therelatively high temperatures employed.

Formation of the bicyclotetrasilazanes of this invention from thetris-(amino)silane has been found to proceed via amino-cyclodisilazanesof the general formula:

wherein R and R are as hereinbefore defined. These novelaminocyclodisilazanes are formed at temperatures lower than thoserequired for the preparation of the corresponding bicyclotetrasilazanesbut above C. For example, the cyclodisilazane in which R representsmethyl and R the propyl radical can be obtained at temperatures of aboutto C. in the presence of about 0.005 mole of acidic catalyst per mole oftris-(amino)silane. When R represents the methyl radical and R the ethylradical, the cyclodisilazane can be obtained at about 140 C. in thepresence of a similar proportion of the catalyst. Such cyclodisilazanescan be isolated and are also included within the scope of thisinvention. The most readily isolated and, therefore, preferredcyclodisilazanes are those in which R is selected from methyl, ethyl andphenyl radicals and R is selected from ethyl and n-propyl radicals.

The cyclodisilazanes can be prepared and isolated and thereafter heated,preferably in the presence of an acid catalyst, to provide thecorresponding bicyclotetrasilazanes. In general, however, no advantageis seen in interrupting the process in this manner. Also formed asintermediate products during the process are compounds of the formula[RSi(NHR)NR'] If desired, these cyclotrisilazanes can also be isolatedand employed as starting materials for the production of the novelbicyclotetrasilazanes. A further aspect of this invention, therefore,resides in a process for the preparation of a bicyclotetrasilazane,which comprises heating under substantially anhydrous conditions and toa temperature above 210 C. an aminocyclosilazane of the general formulawherein x has a value of 2 or 3 and R and R are as hereinbefore defined.

The bicyclotetrasilazanes of this invention can exist in three isomericforms depending on the relative positions of the NHR' and --R groupsattached to each of the two silicon atoms. Preparation of the compoundsaccording to the process described herein leads to a mixture of allthree isomeric forms. The bicyclotetrasilazanes are useful as startingmaterials for the preparation of silazane polymers as intermediates inthe preparation of silazanesiloxane copolymers and as cross-linkingagents for siloxane polymers. 7

The following examples illustrate the invention.

EXAMPLE 1 Methyl-tris-(ethylamino)silane (33.0 g.) was heated at 140 C.for 8 hours in the presence of ammonium sulphate (0.126 g., 0.005 molarratio) in an atmosphere of dry argon, the ethylamine evolved beingallowed to escape to the atmosphere. The pyrolysate was fractionallydistilled at reduced pressure yielding (after removal of the unreactedstarting material) dimethyl-1,3-diethyl-2,4-bis-(ethylamino)-cyclodisilazane [MeSi(NHEt)NEt] (7.9 g., 32%) a colorlessliquid B.P. 33/0.05 mm. 11 1.4416.

Calcd. for C H N Si (percent): C, 46.2; H, 10.8; N, 21.5; Si, 21.5; M,260. Found (percent): C, 46.3; H, 10.7; N, 20.3; Si, 21.3; M (by massspectrum) 260.

The structure of the product was confirmed by infrared and NMR (NuclearMagnetic Resonance) spectroscopy.

EXAMPLE 2 Methyl-t-ris-(ethylamino)silane (267 g.) was gradually heatedup to 345 over a period of 97 hours in the presence of sulphuric acid(1.16 g., 0.0078 molar ratio) in an atmosphere of dry argon, ethylaminebeing evolved. Fractional distillation of the pyrolysate at reducedpressure yielded N-pentaethyl-bis- (ethylaminotetramethylbicyclotetrasilazane Me Si (NEt) (NHEt) (66.2 g.; 36%) B.P.140-147/0.6 mm. which solidified to colorless waxy crystals M.P. 84.

Calcd. for C H N Si (percent): C, 45.4; H, 10.3; N, 20.6; Si, 23.6; M,475. Found (percent): C, 45.4; H, 10.5; N, 20.8; Si, 23.8; M (by massspectrum 475).

The structure of the product was confirmed by infrared and NMR (NuclearMagnetic Resonance) spectroscopy.

EXAMPLE 3 Methyl-tris-(methylamino)silane (112 g.) was heated in thepresence of sulphuric acid (0.166 g., 0.002 molar ratio) in anatmosphere of dry argon. The temperature was raised to 210 graduallyover a period of 120 hours and maintained at between 210 and 250 for afurther 140 hours. Methylamine was evolved rapidly at first and thenmore slowly. The composition of the pyrolysate was analyzed at intervalsby gas chromatography and showed a ratio of bicyclotetrasilazane tocyclotrisilazane of approximately 3:2 at the end of the above pyrolysis.The pyrolysate mixture could not be separated by distillation in thiscase as the two major products were crystalline solids. Preparative gaschromatography was used to obtain a pure sample ofN-pentamethyl-bis-(methylamino) tetramethylbicyclotetrasilazane, acolorless crystalline solid M.P. l35150.

Calcd. for C11H35N1Si4 (percent): C, 35.0; H, 9.3; N, 26.0; Si, 29.7; M,377. Found (percent): C, 34.9; H, 9.3; N, 25.4; Si, 30.2; M (massspectrum) 377.

The structure was confirmed by infrared and NMR (Nuclear MagneticResonance) spectroscopy.

EXAMPLE 4 N trimethyltriphenyltris (methylamino)cyclotrisila- Zane (14.1g.) (prepared from the acid catalyzed pyrolysis ofphenyltris-(methylamino)silane) was heated at 300 to 330 for 282 hoursin the presence of sulphuric acid (0.0093 g., 0.0033 molar ratio) in adry argon atmosphere. Methylamine was evolved and gas chromatographyanalysis showed that the product contained some unreacted startingmaterial andN-pentamethyl-bis-(methylamino)tetraphenylbicyclotetrasilazane, acolorless solid which was isolated by repeated fractional sublimation(1.7 g., 13%).

Calcd. for C H N Si (percent): C, 59.5; H, 6.9; N,

4 15.7; Si, 17.9. Found (percent): C, 59.4; H, 7.3; N, 14.8; Si, 18.4.

EXAMPLE 5 When vinyltris-(methylamino)silane was heated in the presenceof sulphuric acid in an atmosphere of dry argon and temperature wasraised to 210 C. gradually over a period of hours and maintained atbetween 210 C. and 250 C. for a further hours, equivalent results wereobtained.

That which is claimed is:

1. A novel composition of matter comprising bicyclotetrasilazanes of thegeneral formula:

wherein each R represents an alkyl. radical containing less than 6carbon atoms, a monocyclic aryl radical, a vinyl radical or an allylradical and each R represents an alkyl radical containing less than 6carbon atoms or an allyl radical.

2. A novel composition of matter as claimed in claim 1 wherein each Rrepresents a methyl, ethyl or phenyl radical and each R represents amethyl or ethyl radical.

3. A novel composition of matter as claimed in claim 1 wherein thebicyclotetrasilazane is N-pentaethylbis-(ethylamino)tetramethylbicyclotetrasilazane.

4. A novel composition of matter as claimed in claim 1 wherein thebicyclotetrasilazane is N-pentamethylbis-(methylamino)tetramethylbicyclotetrasilazane.

5. A novel composition of matter as claimed in claim 1 wherein thebicyclotetrasilazane is N-pentamethylbis- (methylaminotetraphenylbicyclotetrasilazane.

6. A process for the preparation of a novel bicyclotetrasilazane of thegeneral formula:

RN NR NR S i NSiR i I R R NHR' which comprises heating undersubstantially anhydrous conditions and to a temperature above 210 C. atris- (amino)-silane of the formula RSi(NHR) wherein each R representsan alkyl radical containing less than 6 carbon atoms, a monocyclic arylradical, a vinyl radical or an allyl radical and each R represents analkyl radical colntaining less than 6 carbon atoms or an allyl radica 7.A process for the preparation of a novel bicyclotetrasilazane as claimedin claim 6 wherein each R represents a methyl, ethyl or phenyl radicaland each R represents a methyl or ethyl radical.

8. A process for the preparation of a novel bicyclotetrasilazane asclaimed in claim 6 wherein the tris- (amino) silane is heated in thepresence of a catalyst which functions as a proton source at thereaction temperature.

9. A process for the preparation of a novel bicyclotetrasilazane asclaimed in claim 8 wherein the catalyst is sulphuric acid or ammoniumsulfate.

10. A process for the preparation of a novel bicyclotetrasilazane of thegeneral formula 5 which comprises heating under substantially anhydrousReferences Cited conditions and to a temperature above 210 C. an amino-UNITED STATES PATENTS cyclosilazane of the general formula [RSi(NHR')NR'] wherein x has the value of 2 or 3 and wherein each R repg fwgg-ggg 386522 t 1 6 b r r resents an alkyl radical con annng ess than caron 5 3,414,584 12/1968 Fink 260*448'2 atoms, a monocyclic aryl radical,a vinyl radical or an allyl radical and each R represents an alkylradical containing less than 6 carbon atoms or an allyl radical. HELENMCCARTHY Pnmary Exammer 11. A process for the preparation of a novelbicyclo- W. F. W. BELLAMY, Assistant Examiner tetrasilazine as claimedin claim 10 wherein each R rep- 10 U 8 Cl X R resents a methyl, ethyl orphenyl radical and each R rep- 260 46 5 448 2 resents a methyl or ethylradical.

